I've not come across the word before so it's time for the giant Collins Robert. But the book has little to say, giving only the definition, "bully boy". Matamore can be used as a verb phrase, "faire le matamore", meaning to throw one's weight around.
There's also a note relating the word to "fanfaron" which is an adjective meaning, "boastful".
I have to admit that I have always thought that Brick Bradford was Luc Bradefer in French.
So we're really no further forward.
Re thick as a brick, here we also say, thick as mince and, a bit rude, thick as the s***e at the neck of a sauce bottle. Sorry to bring the tone of the conversation down.
Just occurred to me, La Patrie was printed during Nazi occupation (the indicia must be on the excised pages so we don't know where it was published or whether there is a censor's stamp on it, as was the case with some comics published during W.W.2) and by that time, the publishers might have needed a new name for Brick/Luc because of Nazi censorship.
Starman's site has no mention of Le Matamore. Interesting. Hmmm.